Wing tip with optimum loading

ABSTRACT

A wing incorporating a wing tip with a curved leading edge and a curved trailing edge to minimize induced drag for a given wing form is disclosed. The curve of the leading and trailing edges of the wing tip may be described generally as parabolic, elliptic, or super elliptic. A finite tip segment may be included with a sweep angle between the end of the curved leading edge and the end of the curved trailing edge. The wing tip may also include a spanwise camber.

PRIORITY

This application claims priority to U.S. Provisional Patent ApplicationNo. 61/074,395, filed Jun. 20, 2008, entitled “Curved Wing Tip,” whichis incorporated by reference in its entirety into this application.

BACKGROUND

All aircraft wings experience drag as they move through the air. Theexperienced drag may be separated into three components: induced drag,parasitic drag, and compressibility drag. Induced drag depends on thelift force carried by the lifting surfaces. Parasitic drag arises fromcontact between a moving surface and the fluid and includes such factorsas the object form, skin friction, and interference factors.Compressibility drag is the drag associated with higher Mach numbers,which may include viscous and vortex drag, shock-wave drag, and any dragdue to shock-induced separations, all of which may vary with Machnumber. Of these, the induced drag has traditionally shown the greatestpotential for improvement through the use of winglets or other wing tipdevices.

Generally, an aircraft's wing may be swept to reduce drag effects onhigh-speed airplanes. A swept wing is generally designed so the anglebetween the aircraft's body and the wing is oblique, and specifically isswept toward the aft of the aircraft. The sweep of the wing's leadingedge and trailing edge does not necessarily have to be at the sameangle. A wing tip device may also be added to further reduce the drag onthe wing. One alternative is to provide a raked wing tip. A raked wingtip conventionally has a higher degree of sweep than the rest of thewing. Winglets are also an alternative solution, generally used toincrease the effective aspect ratio of a wing, with less structuralimpact than adding wingspan. Winglets are generally near verticalextensions of the wing tip. Wing tip devices may increase the liftgenerated at the wing tip, and reduce the induced drag caused by wingtipvortices, improving the lift-to-drag ratio.

Various wing tip devices and geometries are described, for example, inU.S. Pat. Pub. No. 2007/0252031 (titled “Wing Tip Devices,” publishedNov. 1, 2007), U.S. Pat. Pub. No. 2007/0114327 (titled “Wing LoadAlleviation Apparatus and Method,” published May 24, 2007), U.S. Pat.No. 6,722,615 (titled “Wing Tip Extension for a Wing,” issued Apr. 20,2004), U.S. Pat. No. 6,827,314 (titled “Aircraft with Active Control ofthe Warping of Its Wings,” issued Dec. 7, 2004), U.S. Pat. No. 6,886,778(titled “Efficient Wing Tip Devices and Methods for Incorporating suchDevices into Existing Wing Designs,” issued May 3, 2005), U.S. Pat. No.6,484,968 (titled “Aircraft with Elliptical Winglets,” issued Nov. 26,2002), U.S. Pat. No. 5,348,253 (titled “Blended Winglet,” issued Sep.20, 1994), each of which is incorporated by reference into thisapplication as if fully set forth herein.

SUMMARY

Embodiments described herein may be applied to a wing incorporating atip device with a curved leading edge and a curved trailing edge tominimize induced drag for a given wing form. The curved leading edge isdesigned to achieve optimal results such as, for example, maintainingattached flow, minimizing flow separation, and minimizing prematurevortex roll-up, while the curved trailing edge is designed to achieveoptimal results such as, for example, keeping the chord distributionconsistent with an elliptic loading over the planform. The curve of theleading and trailing tip sections may be described generally asparabolic, and preferably as super elliptic. A finite tip segment may beincluded with a sweep angle approximate to the trailing edge sweepangle. This finite section may be used to assist in stabilizing the tipvoracity and maintain the vortex position close to the extreme wing tip.

Aerodynamic loading may be important to achieving optimum wingperformance; however, the effect of the actual loading obtained inflight at a wing tip is usually overlooked. Failure to achieve theoptimum elliptic loading, particularly near the tip of the wing, maylead to premature tip vortex formation and a corresponding increase ofinduced drag. This characteristic may also apply to planar wings wherepremature tip vortex roll-up, inboard of the wing tip, is frequentlyvisible as a condensation trail in flight.

Embodiments described herein may be applied to the tip of a flat wing orto a winglet. However, aspects of the design may be applied to liftingsurfaces in general, and particularly to dynamic lifting surfaces.Alternatively, aeronautical propulsion systems, including, for example,propellers and helicopters rotors, may alternatively benefit equallyfrom aspects of the design and are additionally considered within thescope of the invention. Embodiments described herein may also apply toany applications which use either static or dynamic lifting surfacessuch as ship propellers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an exemplary airplane including wing tipgeometry according to embodiments described herein.

FIG. 2A is an enlarged top view of an exemplary wing tip according toaspects of the embodiments described herein.

FIG. 2B is a cross-sectional view of the wing tip of FIG. 2A, takenalong line P-P.

FIG. 3 is an enlarged trailing-edge perspective view of an exemplarywing tip according to embodiments described herein, including a spanwisecamber.

FIG. 4A is an enlarged top view of another exemplary wing tip accordingto aspects of the embodiments described herein.

FIG. 4B is a cross-sectional view of the wing tip of FIG. 4A, takenalong line M-M.

FIG. 5 is an enlarged trailing-edge perspective view of an exemplarywing tip according to embodiments described herein, including a spanwisecamber.

FIG. 6A is a perspective view of an exemplary winglet according toaspects of the embodiments described herein.

FIG. 6B is a trailing-edge perspective view of the winglet of FIG. 6A,including a spanwise camber.

FIG. 7 is a perspective view of an airplane with exemplary propellerrotors according to embodiments described herein.

FIG. 7A is an enlarged view of a propeller blade tip of FIG. 7,according to embodiments described herein.

FIG. 8 is a perspective view of a helicopter with exemplary rotorsaccording to embodiments described herein.

FIG. 8A is an enlarged view of a helicopter rotor tip of FIG. 8,according to embodiments described herein.

DETAILED DESCRIPTION

The following description should be read with reference to the drawings,in which like elements in different drawings are identically numbered.The drawings, which are not necessarily to scale, depict selectedembodiments and are not intended to limit the scope of the invention.For instance, edges appearing be pointed in the drawings may inactuality be rounded (e.g., leading edges in FIG. 2B, FIG. 3, FIG. 4B,and FIG. 5). The description illustrates by way of example, not by wayof limitation, the principles of the invention. This description willclearly enable one skilled in the art to make and use the invention, anddescribes several embodiments, adaptations, variations, alternatives anduses of the invention, including what is presently believed to be thebest mode of carrying out the invention.

As used herein, the terms “about” or “approximately” for any numericalvalues or ranges indicate a suitable dimensional tolerance that allowsthe part or collection of components to function for its intendedpurpose as described herein. Embodiments described herein include anoptimum wing tip geometry for wings. The described geometry may reduceinduced drag associated with premature tip vortex formation fromsuboptimum aerodynamic loading. Embodiments of the tip design geometrymay preserve an elliptic load distribution to the tip of the wing. Inaddition, the airfoil sections may be cambered and twisted to avoid flowseparation along the highly swept leading edge, thereby maintaining anelliptic loading to the extreme tip. Spanwise camber of the liftingsurface may also be incorporated to maintain flow attachment and avoidpremature roll-up of the tip vorticity.

Based on aerodynamic analysis of embodiments incorporating aspects ofthe design described herein, it has been shown that significantreductions of induced drag can be expected relative to that found intypical current tip designs. These results may depend upon the properselection of parameters that define the tip geometry and are discussedbelow. The potential benefits may be expected in the range of about −1%to about −3% induced drag reduction for a commercial transport airplaneconfiguration. The induced drag reduction may correspond to an increasein fuel efficiency in the range of about 0.7% to about 2% at normalcruising speeds. Additional benefits may be expected for low-speedoperation.

Although embodiments of the invention are typically described in termsof wing tip devices, or winglets, the invention is not so limited.Aspects of the invention may be applied to lifting surfaces in general,and particularly to wings generally, and more particularly to aircraft,including planar wings without the use of winglets. Aeronauticalpropulsion systems, including, for example, propellers and helicopters,may alternatively benefit equally from aspects of the invention and areadditionally considered within the scope of the invention. Embodimentsof the present invention may also apply to any applications which useeither static or dynamic lifting surfaces such as helicopters, shippropellers, etc. Finally, other applications that may benefit fromaspects of the invention include devices intended to move air or fluid,such as fans.

Reference axes are used generally to orient the description, as known inthe art, and therefore include a reference system for the airplanegenerally, a reference system for the wing of the airplane, and finallya reference system for the wing tip device. As seen in FIG. 1, anairplane reference system is used to orient the description with respectto the aircraft generally. The x-axis runs along the longitudinal axisof the airplane from nose to the tail. The y-axis is perpendicular tothe x-axis and emanates horizontally from the airplane. Finally, thez-axis is orthogonal to both the x and y axes, oriented in the verticaldirection. A wing reference system may also be used which may differfrom the airplane reference system, as it lies in the reference plane ofthe wing. Therefore, as seen in FIGS. 3 and 5, the reference system maybe rotated by the incidence angle, the dihedral angle, and the sweepangle. Finally, for embodiments described herein, as seen in FIGS. 2 and4, the origin of the reference system for the wing tip geometry iscoincident with the start of the wing tip geometry and lies generally inthe plane of the wing, at the wing tip. However, this relationship maychange substantially from the wing reference system in cases of wingletapplications (e.g., see FIG. 6). The wing tip device reference systemhas its origin at the edge of the wing tip device, the beginning of thecurved leading edge, or curved trailing edge, whichever is closest tothe airplane. The x′-y′ axis is then in the plane of the wing at theorigin of the wing tip. Therefore, the wing tip geometry may be rotatedfrom the airplane reference system by the sweep angle, dihedral angle,incidence angle, and winglet angle, and displaced from the airplanereference system by the length of the wing to the wing tip.

FIG. 1 illustrates an exemplary airplane 102 including a wing tipsection 100 according to embodiments described herein. The wing tipsection 100 may be designed to minimize induced drag by preserving anelliptic load distribution. The leading edge 104 of the wing tip section100 may be curved to avoid flow separation. The trailing edge 106 of thewing tip section 100 may be curved to maintain a desired chordvariation. The trailing edge tip segment 108 may have a small but finitedimension and sweep. The tip segment 108 may be swept at approximatelythe same or similar angle as the trailing edge sweep angle. This tipsegment 108 may assist to stabilize the tip voracity and maintain itsposition at the trailing edge.

Wing 110 has a leading edge 112 and a trailing edge 114. The leadingedge 112 may be substantially straight, and may transition into thecurved leading edge 104 of the wing tip 100. The trailing edge 114 maybe substantially straight before transitioning into the curved trailingsection 106 of the wing tip 100. The leading edge 112 and trailing edge114 may also be swept. However, the leading edge 112 and trailing edge114 may be swept at different angles. For example, the leading edge 112may have a greater sweep angle than the trailing edge 114.

FIG. 2A is a top view that illustrates an exemplary wing tip geometry200 according to embodiments described herein. An x′-y′ reference systemfor the wing tip geometry 200 may be created by a line parallel 202 andanother perpendicular 204 to the longitudinal axis of the airplane body.This x′-y′ reference plane is within the plane of the wing 210. In oneembodiment, the wing 210 is not perpendicular to the plane body, but isswept backwards towards the rear of the airplane. The wing 210 may alsobe rotated upward at a dihedral angle, or tilted about a pitch axis ofthe airplane to create an incidence angle. For example, the leading edge212 may be swept at an angle, Λ_(LE), 214, measured from they′-reference axis 204. The trailing edge 216 may also be swept at anangle, Λ_(TE), 218, also measured from the y′-reference axis 204. Thesweep angle of the leading edge 214 and the trailing edge 218 may be atthe same angle or different angles. Preferably, the sweep angle of theleading edge 214 is greater than the sweep angle of the trailing edge218.

In one embodiment, the tip of the wing 210 has a wing tip geometry 200that curves aft. The wing tip geometry 200 begins along the leading edgeat point 224 and along the trailing edge at point 226. These startinglocations do not necessarily have to be at the same distance away fromthe airplane body. For example, the transition to the curved trailingedge 222 may begin closer to the airplane body than for the curvedleading edge 220. The curved leading edge 220 and the curved trailingedge 222 begin tangential with the leading edge 212 and the trailingedge 216, respectively, and then curve rearward. The curved leading edge220 and curved trailing edge 222 transition smoothly from thesubstantially straight leading edge 212 and trailing edge 216,respectively, and then slopes rearward along a parabolic curveapproaching the freestream velocity direction U 230. Preferably, thecurved leading edge 220 may be closer to approaching the direction ofthe freestream velocity U 230 at the leading edge tip 228, than thecurved trailing edge 222 does at the trailing edge tip 232.

In one embodiment, an end segment 234, between the leading edge tip 228and the trailing edge tip 232, may be located aft of the curved trailingedge 222. This segment 234 may have a specified length and may be sweptat an angle close to or equal to the wing trailing edge sweep angle 218.The preferred length of BD 234 is in the range of 0.15<BD/h<0.20 withthe ratio tending higher at higher values of tip lift coefficients. Itis believed that a segment such as BD 234 is advantageous to stabilizethe tip vortex.

The wing tip section 200 may be applied to a conventional flat wingwhere the curved leading edge 220 and the curved trailing edge 222 maylie in the wing reference plane, i.e., the x′-y′ plane. Therefore, theentire wing, emanating from the plane and terminating at end segment234, are substantially in the same plane. In an alternate embodiment,the wing tip section 200 may be applied to a conventional winglet, wherethe wing end projects out of the x′-y′ reference plane, in thez′-direction. The wing tip section 200 may be integrally formed with therest of the wing 210, or may be a separate section attached or adheredto the tip of the wing. The wing tip section may be attached by bolting,welding, or other known practices of attaching wing segments.

FIG. 2B is a cross-sectional view of the wing tip section 200, atreference P-P of FIG. 2A. A chord, represented by dotted line 236, isthe line from the curved leading edge 220 to the curved trailing edge222, which may be at an angle θ_(T) relative to the x′-reference line202. The chord distribution conforms to the optimum aerodynamic loadingon the wing surface. In one embodiment, the curved trailing edge 222 isdesigned to maintain a desired chord distribution to achieve ellipticloading. Airfoil sections may also be incorporated at specifiedlocations, y′, corresponding to the local chord line and the twist angleθ_(T) distribution.

These features may be alternatively described in mathematical terms,where all dimensions may be normalized relative to the tip extensionlength, g 250. The tip extension length, g 250 is the straight-linedistance of the trailing edge that extends past the trailing edge originof the wing tip geometry. Therefore, this distance is the x′-directiondifference between the origin of the wing tip along the trailing edge226 and the very end 228 of the curved leading edge 220. Anotherreference length h 252 is provided corresponding to the height of theextension length, and is the y′-distance from the curved wing tipsection 200 origin along the trailing edge, point 226, to the trailingedge end point 232. Points A, B, C, D, and E are added for referencelocations. Point A 224 is the point where the leading edge 212transitions into the curved leading edge 220, and deviates from the linetangential with the leading edge 212. Reference point C 226 is thecorresponding point along the trailing edge 216. Point B 228 is the endof the curved leading edge 220, while point D 232 is the end of thecurved trailing edge 222. The segment BD is the end segment 234.

The curved leading edge 220, from point A to C, may be defined by:x−x _(A)=tan Λ_(LE)*(y−y _(A))+M ₁*(y−y _(A))^(m) ¹ +M ₂*(y−y _(A))^(m)² .

In one embodiment, the curved leading edge parameters M₁ and m₁, M₂ andm₂ are selected to define a planform that may maintain attached flow andavoid flow separation. The parameters are therefore chosen to create asmooth parabolic transition from the substantially straight leading edge212 to the desired end slope at point B 228. The desired leading edgeslope, dy/dx, at point B approaches the freestream direction U 230 andmay be in the range of about 0.0 to about 0.1, and is preferably in therange about 0.03 to about 0.07. In one embodiment, the desired leadingedge slope approaches about 0.05. To provide optimal performancecharacteristics, where x_(A), y_(A), g, and Λ_(LE) are given, M₁ is inthe range of about 0.4 to about 0.6, M₂ is in the range of about 0.08 toabout 0.12, m₁ is in the range of about 3.6 to about 5.4, and m₂ is inthe range of about 5.2 to about 7.7. Preferably, M₁ is about 0.5, M₂ isabout 0.1, m₁ is about 4.5, and m₂ is about 6.5. The inclusion of twopower terms is preferred to sufficiently provide control of the leadingedge slope, dy/dx at point B and to match the optimum leading edge curveshape. The design includes at least one power term to create the smoothparabolic transition from the leading edge to the end point. However,power terms may be removed or added to further approach the optimumperformance.

The curved trailing edge 222, from point C to D, may be defined by:x−x _(C)=tan Λ_(TE)*(y−y _(C))+N ₁*(y−y _(C))^(n) ¹ +N ₂*(y−y _(C))^(n)² .

In one embodiment, the curved trailing edge parameters N₁ and n₁, N₂ andn₂ are selected to maintain an appropriate chord variation and controlof the trailing edge slope, dy/dx, approaching point D. The parametersare chosen to provide a smooth parabolic transition from thesubstantially straight trailing edge to achieve elliptic loading overthe wing tip form. The parameters may additionally be chosen to controlthe trailing edge slope at point D to approach the freestream direction.For example, the trailing edge slope at point D may fall within therange of about 0.0 to about 2.0. In one embodiment, the trailing edgeslope approaching point D is in the range of about 0.06 to about 0.15,and is preferably about 0.10. To provide optimal performancecharacteristics, where x_(C), y_(C), segment BD, h, and Λ_(TE) aregiven, N₁ is in the range of about 0.08 to about 0.12, N₂ is in therange of about 0.16 to about 0.24, n₁ is in the range of about 2.8 toabout 4.2, and n₂ is in the range of about 3.6 to about 5.4. Moreparticularly, N₁ is about 0.1, N₂ is about 0.2, n₁ is about 3.5, and n₂is about 4.5. The inclusion of two power terms are preferred tosufficiently control the loading on the wing tip and maintain anappropriate chord variation. However, fewer or additional power termsmay be removed or added to more particularly control these features. Atleast one power term should remain to achieve the parabolic transitionfrom trailing edge to tip.

The end segment BD, may have a small but finite dimension and may beswept at an angle approximate to the trailing edge angle, Λ_(TE), 218.This section may assist in stabilizing the tip voracity and maintain thevortex position very close to the extreme tip, at point B. The length ofsegment BD may be determined from the other parameters alreadydescribed.

The airfoil sections may be cambered or twisted to maintain an ellipticloading to the wing tip and avoid flow separation along the curvedleading edge 220. The wing chord, the reference line between the curvedleading edge 220 and the curved trailing edge 222, may be designedaccording to the parameters above to maintain the desired chorddistribution. The airfoil may additionally be twisted, thereby anglingthe chord relative to the free stream direction θ_(T). Airfoil twist maybe defined by the rotation angle of the airfoil chord about the tiptrailing edge, CDB, relative to the wing reference plane, x′-y′ plane.In alternate embodiments, the airfoil shapes may be modified versions ofthe basic winglet airfoil.

FIG. 3 is a profile view of the wing 310 from the trailing edge,illustrating a representative wing tip section 300 according to aspectsof the embodiments described herein, including a spanwise camber. Thespanwise camber may be generated by displacement in the z′-direction, ofthe wing tip section 300 curved trailing edge 322 from the wingreference plane, x′-y′. In one embodiment, this curve, lying in they′-z′ plane, emanates tangentially from the wing trailing edge 316 atpoint 326 and deviates parabolically to terminate at trailing edge endpoint 332. Therefore, the end of the wing tip section 300 curves out ofthe x′-y′ reference plane starting at the origin of the curved trailingedge 322 at point 326. The path of the trailing edge out of thereference plane, which is generally parabolic in the y′-z′ plane,generates a wing tip surface that may be substantially cylindrical untilthe trailing edge terminates at point 332. In another embodiment, thespanwise camber creates part of a cylindrical surface, which may beaugmented by superposition of airfoil camber and twist.

The representative profile of a wing according to aspects of embodimentsdescribed herein, including a spanwise camber, may alternatively bedescribed in mathematical terms. The wing may include a slight inclineθ_(D), the dihedral angle 350, from horizontal 352, as the wingapproaches the wing tip section 300. In one embodiment, the wing tipsection 300 may also, or alternatively incorporate a spanwise camber tomaintain flow attachment, reduce flow separation, and minimize prematureroll-up along the outer edge of the tip design.

The camber may be defined in terms of vertical displacement, z, of thecurved trailing edge CD from a straight line extension of the wingtrailing edge, y′-axis, and may be defined by:z−z _(C) =−P*(y−y _(C))^(p), where y _(C) <y<y _(D).

In one embodiment, the parameters P and p, are selected in combinationwith the wing incline and twist to define the lifting surface betweenthe previously defined curved leading and curved trailing edges. Toprovide optimal performance characteristics, where x_(C), y_(C), segmentBD, h, and Λ_(TE) are given, P is in the range of about 0.12 to about0.18, and p is in the range of about 2.0 to about 3.0. Preferably, P isabout 0.15, and p is about 2.5. Alternatively, the design may includethe wing tip section 300 curved in the opposite direction, or in thepositive z direction, according to the same principles.

The above combination of parameters to maintain the elliptic loading andattached flow to the tip may be defined in relation to the wing planform(i.e., sweep and taper) and aerodynamic loading. The design parametersmay be specified within appropriate limits to provide optimalperformance characteristics.

FIG. 4A represents an alternate embodiment and illustrates a top view ofa wing 410, including a wing tip section 400 according to embodimentsdescribed herein. The wing 410 includes a substantially straight leadingedge 412, swept rearward at an angle Λ₁ 414 and a trailing edge 416,which is also substantially straight and swept rearward at an angle Λ₂418.

In one embodiment, the wing tip section 400 starts at point 424 alongthe leading edge 412, and point 426 along the trailing edge 416. Thestarting points 424 and 426 may occur at the same distance away from theairplane body, as shown, or may occur at different distances. Forexample, the transition point 424 may occur as shown, but the transitionalong the trailing edge 416 may be further away from the airplane body.The wing tip section 400 includes a curved leading edge 420 and curvedtrailing edge 422. The curved leading edge 420 emanates from a linetangential with the leading edge 412 and smoothly transitions along anelliptical curve to an endpoint 428, where the slope approaching point428 approaches the freestream direction U. Similarly, the curvedtrailing edge 422 emanates tangentially from the trailing edge 416 andcurves rearward along an elliptic curve to an endpoint 432, where theslope approaching the end point 432 also approaches the freestreamdirection U. The slope approaching the wing tip is not necessarily thesame along the curved leading edge 420 and the curved trailing edge 422.

In one embodiment, an end segment 434, between the leading edge tip 428and the trailing edge tip 432, may be located aft of the curved trailingedge 422. This segment 434 may have a specified length and may be sweptat an angle close to or equal to the wing trailing edge sweep angle 418.The preferred length of CE 434 is in the range of 0.15<CE/h<0.20 withthe ratio tending higher at higher values of tip lift coefficients. Itis believed that a segment such as CE 434 is advantageous to stabilizethe tip vortex.

This embodiment may also be described in mathematical terms to define anoptimum design that maintains attached flow and avoids premature vortexroll-up. Points A, B, C, D, and E along with lengths c₁, c₂, g, and hhave been included for reference. Point A 424 and D 426, are the originof the curved end section 400 along the leading edge 412 and trailingedge 416 respectively. Points C 428 and E 432 are the respective end tiplocations of the curved leading edge 420 and curved trailing edge 422,respectively. Point B is the point along the curved leading edge 420corresponding to the same y′-distance as point E of the curved trailingedge 422. Reference length c₁ is the distance along the x′-directionbetween reference points A and D; while reference length c₂ is thedistance along the x′-direction between points B and E. Reference heighth is the y′-direction distance from the origin along the trailing edge,point D, to the curved wing tip extreme end, point C. Reference distanceg is the x′-direction distance from the same origin as the h referencedistance, point D, to the curved leading edge end point C.

The leading curved edge 420, from point A to C, may be defined by:x−x _(A) =[y−y _(A)]*tan Λ₁ +a ₁*[(1−([y−y _(A) ]/b ₁)^(n) ¹ )^((1/m) ¹⁾−1].

In one embodiment, the curved leading edge geometry parameters a₁, b₁,m₁, and n₁ are selected to define a planform that maintains attachedflow and reduces flow separation, while minimizing premature vortex rollup. The inclusion of these four parameters are sufficient to providecontrol of the leading edge curvature near point A, and the contourslope at point C, to define the optimum leading edge contour. Additionalterms may, however, be added or removed to further refine the optimumparameters.

The sizing parameters (g/c₁), (h/c₁), (dy/dx)_(c), and (c₂/c₁) relate tooverall planform proportions and provide the framework for the optimumcontours for both leading edge and trailing edge. To provide acceptableperformance levels, (g/c₁) is in the range of about 0.50 to about 0.80,(h/c₁) is in the range of about 0.60 to about 1.00, (dy/dx)_(c) is inthe range of about 0.03 to about 0.07, and (c₂/c₁) is in the range ofabout 0.60 to about 0.70. In one embodiment, (g/c₁) is about 0.60,(h/c₁) is about 0.70, (dy/dx)_(c) is about 0.05, and (c₂/c₁) is about0.65.

The leading edge contour parameters (a₁/c₁), (b₁/c₁), m₁, and n₁ definethe leading edge contour within the sizing framework. To provideacceptable performance levels, (a₁/c₁) is in the range of about 1.50 toabout 2.50, (b₁/c₁) is in the range of about 0.60 to about 0.90, m₁ isin the range of about 2.0 to about 4.0, and n₁ is in the range of about1.50 to about 3.0. In one embodiment, (a₁/c₁) is about 2.0, (b₁/c₁) isabout 0.70, m₁ is about 3.0, and n₁ is about 2.0.

The curved trailing edge 422, from point C to E, may be defined by:x−x _(D) =y*tan Λ₂ +a ₂*[(1−([y−y _(D) ]/b ₂)^(n) ² )^((1/m) ² ⁾−1].

In one embodiment, to provide optimal performance characteristics, thetrailing edge curvature near point D and the contour slope at point Eare defined to achieve the chord distribution consistent with anelliptic loading over the planform to minimize drag.

The sizing parameters (g/c₁), (h/c₁), (dy/dx)_(E), and (c₂/c₁) relate tooverall planform proportions and provide the framework for the optimumcontours for both leading edge and trailing edge. These sizingparameters, with the exception of (dy/dx)_(E), have been previouslyselected, as discussed above in terms of the curved leading edgegeometry. Sizing parameter (dy/dx)_(E) is acceptable within the range ofabout 0.06 to about 0.15, and is preferably about 0.10. Therefore,contour parameters, (a₂/c₁), (b₂/c₁), m₂, and n₂ remain to be selected.The trailing edge contour parameters (a₂/c₁), (b₂/c₁), m₂, and n₂ definethe trailing edge contour within the sizing framework. To provideacceptable performance levels, (a₂/c₁) is in the range of about 0.80 toabout 1.50, (b₂/c₁) is in the range of about 0.30 to about 0.60, m₂ isin the range of about 1.50 to about 2.50, and n₂ is in the range ofabout 1.50 to about 2.50. In one embodiment, (a₂/c₁) is about 1.0,(b₂/c₁) is about 0.40, m₂ is about 2.0, and n₂ is about 2.0.

In one embodiment, the end section, segment CE, has a small but finitedimension and may be swept at the trailing edge angle Λ₂. This sectionmay assist in stabilizing the tip voracity and maintain the vortexposition close to the extreme tip, point E. The length of segment CE isdetermined by sizing and contour parameters.

FIG. 4B illustrates a representative cut-away airfoil section M-M fromFIG. 4A. The airfoil section may be cambered and twisted to maintain anelliptic loading to the extreme tip and avoid flow separation along thehighly swept curved leading edge 420. Airfoil twist may be defined bythe rotation angle of the airfoil chord about the tip trailing edge,CED, relative to the wing reference plane, x′-y′. Airfoil shapes mayalso be modified versions of the basic winglet airfoil.

FIG. 5 illustrates a representative profile of a wing 510, as seenlooking toward the trailing edge 516, according to aspects of the designincluding a representative spanwise camber. The wing may include aslight incline, the dihedral angle, θ_(D), 550, from horizontal 552, asthe wing approaches the curved tip section 500. In one embodiment, thewing tip geometry 500 may also, or alternatively, incorporate spanwisecamber of the wing lifting surface to maintain flow attachment, reduceflow separation, and minimize premature roll-up along the outer edge ofthe tip design.

The camber may be defined in terms of lateral displacement, z, of thecurved trailing edge 522, CD, from a straight line extension of the wingtrailing edge 516, and may be defined by:z/c ₁ =−P*([y−y _(D) ]/h−1)^(p).

In one embodiment, the parameters P and p are selected in combinationwith the airfoil camber and twist and define the lifting surface betweenthe previously defined curved leading and curved trailing edges. Toprovide optimal performance characteristics, where the sizing parametersare given as described above, P is in the range of about 0.10 to about0.25, and p is in the range of about 2.0 to about 4.0. Moreparticularly, P is about 0.15, and p is about 2.5. The spanwise cambermay alternatively curve in the opposite, or positive z, direction.

The above combination of parameters to maintain the elliptic loading andattached flow to the ultimate tip may be defined in relation to the wingplanform (i.e. sweep and taper) aerodynamic loading. The designparameters may be specified within appropriate limits to provide optimalperformance characteristics.

FIG. 6A illustrates a representative wing 610 with wing end section 600according to embodiments described herein as applied to a winglet 660.The end of the wing may be turned upward in a winglet form 660, asillustrated more fully in FIG. 6B. The winglet 660 is attached to theend of the wing 610 and may be of any conventional design. For example,as shown, the winglet 660 transitions 662 out of the plane of the wing610 in a vertical direction. The transition section 662 may be acontinuous transition, as shown, such as along a constant radius,parabolic, or elliptic curve; the transition may alternatively benon-continuous. The end of the winglet 660, after the transition section662, may be substantially straight. The wing 610 may be at an angleθ_(D) 650 from the horizontal 652. The leading edge 612 and trailingedge 616 may be substantially straight within the plane of the wingthrough the transition section 662 until the wing tip section 600. Theleading edge 612 and trailing edge 616 merely transition in a verticaldirection to form the winglet 660.

The winglet 660 may include a curved leading edge 620, a curved trailingedge 622, and an end segment 634. The curved leading edge 620 deviatesfrom the upward turned tangential of the leading edge 612, while thecurved trailing edge 622 deviates from the upward turned tangential ofthe trailing edge 616. The curved leading edge 620 and curved trailingedge 622 may be parabolic or elliptic. The end segment 634 may also bedesigned according to embodiments described herein. The winglet 660 mayalso incorporate aspects of the design for the spanwise camber as shownin FIG. 6B. The curved end section 600 may not encompass the entirewinglet 660, and preferably is only at the end of the winglet after thetransition section 662.

FIGS. 7-7A illustrate a representative airplane 700 including anexemplary propeller 702 according to embodiments described herein. Thepropeller tip geometry may include a curved leading edge 704, a curvedtrailing edge 706 with an end segment 708. The curved sections 704 and706 transition smoothly from the propeller blade body 710. The curvedleading edge 704 may be designed according to embodiments describedherein. The curved leading edge 704 may be parabolic or elliptic anddesigned to maintain attached air flow and reduce flow separation. Thecurved trailing edge 706 may also be designed according to embodimentsdescribed herein, and may follow a parabolic or elliptic contour tomaintain an appropriate chord variation and control the trailing edgeslope at the tip of the propeller. The substantially straight sections712 and 714 may be angled toward the trailing edge away from the radialprojection of the propeller 702 center. End section 708 may connect theend of the curved leading edge 704 and the end of the curved trailingedge 706. The segment may be a finite dimension and angled to stabilizethe tip vorticity and maintain the trailing vortex position at the tip.The design parameters for this application are essentially the same asfor the cases previously illustrated. Similarly, embodiments describedherein may be applied to duel propeller aircraft, where the propellersmay be attached to the aircraft wings.

FIGS. 8-8A illustrate a representative helicopter 800 including anexemplary rotor 802 according to embodiments described herein. The rotortip geometry may include a curved leading edge 804, a curved trailingedge 806, and an end segment 808. The curved sections 804 and 806transition smoothly from the body of rotor 810. The curved leading edge804 may be parabolic or elliptic and designed according to aspects ofthe above described disclosed embodiments to maintain attached air flowand reduce flow separation. The curved trailing edge 806 may also beparabolic or elliptic, but may be designed with different parametersaccording to aspects of the disclosed design to maintain an appropriatechord variation and control the trailing edge slope at the tip of thepropeller. End segment 808 may connect the end of the curved leadingedge 804 and the end of the curved trailing edge 806 according toaspects of the disclosed design. The segment may be a finite dimensionand angled to stabilize the tip vorticity and maintain the trailingvortex position at the tip. The design parameters for this applicationare essentially the same as for the cases previously illustrated.

While the design has been described in terms of particular variationsand illustrative figures, those of skill in the art will recognize thatthe design is not limited to the variations or figures described.Therefore, to the extent there are variations of the invention, whichare within the spirit of the disclosure or equivalent to the inventionsfound in the claims, it is the intent that this patent will cover thosevariations as well. Variations contemplated within the scope of theinvention include combinations of various features of the exemplaryembodiments into new embodiments, as would be apparent to one skilled inthe art. These equivalents may additionally include variousapplications, such as: water craft, other aircraft, or applicationsgenerally intended to move gas or liquid. For example, water craftincluding propellers, helicopters, and propeller airplanes are allunderstood to benefit from one or more embodiments described herein.Alternatively, fans, including ventilation systems, are also understoodto benefit from one or more embodiments described herein. Finally, allpatents, publications, and patent applications cited in this descriptionare herein incorporated by reference in their entirety as if eachindividual publication or patent application were specifically andindividually put forth herein.

What is claimed is:
 1. A curved wing tip connected to a wing of anaircraft, the wing having a wing leading edge and a wing trailing edge,the aircraft in a x′-y′-z′ reference system where x′ is a longitudinalaxis of the aircraft, y′ is a horizontal axis of the aircraft, and z′ isa vertical axis of the aircraft orthogonal to x′ and y′, the curved wingtip in a x-y-z reference system where x is parallel to x′, y is tangentto a surface of the wing, and z is orthogonal to x and y, the curvedwing tip comprising: a leading edge curved to maintain attached air flowand defining a first parabolic line from a leading start point to aleading end point, the first parabolic line approaching a freestreamdirection parallel to the longitudinal axis of the aircraft at theleading end point; a trailing edge curved to maintain elliptic loadingover the wing tip defining a second parabolic line from a trailing startpoint to a trailing end point, the second parabolic line approaching thefreestream direction parallel to the longitudinal axis of the aircraftat the trailing end point; and an end segment connecting the leading endpoint and the trailing end point swept back from the trailing end pointto the leading end point at an end segment angle to maintain a trailingvortex position close to the trailing end point.
 2. The curved wing tipaccording to claim 1, further comprising a winglet portion positionedbetween the wing and the curved wing tip leading start point andtrailing start point, the winglet extending away from the surface of thewing with a vertical component.
 3. A curved wing tip connected to a wingof an aircraft, the wing having a wing leading edge and a wing trailingedge, the aircraft in a x′-y′-z′ reference system where x′ is alongitudinal axis of the aircraft, y′ is a horizontal axis of theaircraft, and z′ is a vertical axis of the aircraft orthogonal to x′ andy′, the curved wing tip in a x-y-z reference system where x is parallelto x′, y is tangent to a surface of the wing, and z is orthogonal to xand y, the curved wing tip comprising: a leading edge curved to maintainattached air flow and defining a first elliptic line from a leadingstart point to a leading end point, the first elliptic line approachinga freestream direction parallel to the longitudinal axis of the aircraftat the leading end point; a trailing edge curved to maintain ellipticloading over the wing tip defining a second elliptic line from atrailing start point to a trailing end point, the second elliptic lineapproaching the freestream direction parallel to the longitudinal axisof the aircraft at the trailing end point; and an end segment connectingthe leading end point and the trailing end point swept back from thetrailing end point to the leading end point at an end segment angle tomaintain a trailing vortex position close to the trailing end point. 4.The curved wing tip according to claim 3, further comprising a wingletportion positioned between the wing and the curved wing tip leadingstart point and trailing start point, the winglet extending away fromthe surface of the wing with a vertical component.
 5. An aircraft havinga curved wing tip connected to an end of a wing, the wing having a wingleading edge and a wing trailing edge, the aircraft having a front, arear, and a longitudinal axis from the front to the rear, the curvedwing tip in a x-y-z reference system where x is parallel to the aircraftlongitudinal axis, y is tangent to a surface of the wing, and z isorthogonal to x and y, the curved wing tip comprising: a leading edgedefining a first curved line from a leading start point to a leading endpoint, the first curved line approaching a freestream direction parallelto the longitudinal axis of the aircraft at the leading end point; atrailing edge defining a second curved line from a trailing start pointto a trailing end point, the second curved line approaching thefreestream direction parallel to the longitudinal axis of the aircraftat the trailing end point, the trailing end point positioned forward theleading end point; and an end segment connecting the trailing end pointto the leading end point swept back at an end segment angle.
 6. Theaircraft according to claim 5, wherein the angle is approximately equalto a sweep angle of the wing trailing edge.
 7. The aircraft according toclaim 5, wherein the first curved line is a first parabolic line and thesecond curved line is a second parabolic line.
 8. The aircraft accordingto claim 5, wherein the first curved line is a first elliptical line andthe second curved line is a second elliptical line.
 9. The aircraftaccording to claim 5, wherein the leading edge of the wing tip is closerto the freestream direction at the leading end point than the trailingedge of the wing tip at the trailing end point.
 10. The aircraftaccording to claim 5, wherein the wing is generally planar and thecurved wing tip extends out of a plane of the wing.
 11. The aircraftaccording to claim 5, wherein the wing is generally planar and thecurved wing tip comprises a generally planar portion.
 12. The aircraftaccording to claim 11, wherein the generally planar portion of thecurved wing tip is angled greater than 0° to the plane of the wing. 13.The aircraft according to claim 12, wherein the generally planar portionof the curved wing tip has a generally linear leading edge and generallylinear trailing edge.
 14. The aircraft according to claim 13, furthercomprising a wing tip end portion curved out of the plane of thegenerally planar portion of the curved wing tip.
 15. The aircraftaccording to claim 5, wherein the leading edge forms a first parabolicline starting generally tangential with the wing leading edge, andwherein the trailing edge forms a second parabolic line startinggenerally tangential from the wing trailing edge, the first parabolicline at the leading end point approaching the freestream direction moreclosely than the second parabolic line at the trailing end point. 16.The aircraft according to claim 15, wherein the first parabolic linedefined by the leading edge meets a criteria:x−x _(A)=tan Λ_(LE)*(y−y _(A))+M ₁*(y−y _(A))^(m) ¹ +M ₂*(y−y _(A))^(m)² , wherein M₁ is in a range of about 0.4 to about 0.6, M₂ is in a rangeof about 0.08 to about 0.12, m₁ is in a range of about 3.6 to about 5.4,and m₂ is in a range of about 5.2 to about 7.7.
 17. The aircraftaccording to claim 15, wherein the second parabolic line defined by thetrailing edge meets a criteria:x−x _(C)=tan Λ_(TE)*(y−y _(C))+N ₁*(y−y _(C))^(n) ¹ +N ₂*(y−y _(C))^(n)² , wherein N₁ is in a range of about 0.08 to about 0.12, N₂ is in arange of about 0.16 to about 0.24, n₁ is in a range of about 2.8 toabout 4.2, and n₂ is in a range of about 3.6 to about 5.4.
 18. Theaircraft according to claim 15, wherein the wing tip includes an airfoilchordwise camber and twist.
 19. The aircraft according to claim 15,wherein the end segment has a ratio of end segment length to height in arange of about 0.015 to about 0.020.
 20. The aircraft according to claim5, wherein the leading edge forms a first elliptic line startinggenerally tangential with the wing leading edge, and wherein thetrailing edge forms a second elliptic line starting generally tangentialfrom the wing trailing edge, the first elliptic line at the leading endpoint approaching the freestream direction more closely than the secondelliptic line at the trailing end point.
 21. The aircraft according toclaim 20, wherein the first elliptic line defined by the leading edgemeets a criteria:x−x _(A) =[y−y _(A)]*tan Λ₁ +a ₁*[(1−([y−y _(A) ]/b ₁)^(n) ¹ )^((1/m) ¹⁾−1], wherein (a₁/c₁) is in a range of about 1.50 to about 2.50, (b₁/c₁)is in a range of about 0.60 to about 0.90, m₁ is in a range of about 2.0to about 4.0, and n₁ is in a range of about 1.50 to about 3.0.
 22. Theaircraft according to claim 20, wherein the second elliptical linedefined by the trailing edge meets a criteria:x−x _(D) =y*tan Λ₂ +a ₂*[(1−([y−y _(D) ]/b ₂)^(n) ² )^((1/m) ² ⁾−1],wherein (a₂/c₁) is in a range of about 0.80 to about 1.50, (b₂/c₁) is ina range of about 0.30 to about 0.60, m₂ is in a range of about 1.50 toabout 2.50, and n₂ is in a range of about 1.50 to about 2.50.
 23. Theaircraft according to claim 20, wherein the wing tip includes an airfoilchordwise camber and twist.
 24. The aircraft according to claim 20,wherein the wing tip is defined by a set of sizing parameters toconstrain an overall proportion of the wing tip, including: a wing tipheight (h) measuring a distance radially away from a plane body from theleading start point to the leading end point; a wing tip length (g)measuring a distance parallel to the plane body from the leading startpoint to the leading endpoint; an initial cord length (c₁) measuring acord length from the leading start point to the trailing start point;and an end cord length (c₂) measuring a cord length from the leadingedge to the trailing edge parallel to the plane body at the trailing endpoint; wherein (g/c₁) is in a range of about 0.50 to about 0.80, (h/c₁)is in a range of about 0.60 to about 1.00, and (c₂/c₁) is in a range ofabout 0.60 to about 0.70.
 25. The aircraft according to claim 24,wherein a leading edge slope at the leading end point is in a range ofabout 0.03 to about 0.07, and a trailing edge slope at the trailing endpoint is in a range of about 0.06 to about 0.15.
 26. The aircraftaccording to claim 20, wherein the end segment has a ratio of endsegment length to height in a range of about 0.015 to about 0.020.